Components of gas turbine engines are exposed to an environment containing high temperature, high pressure, high velocity combustion gases containing moisture, oxygen and other corrosive compounds. Modern gas turbine engines may have firing temperatures that exceed 1,600° C., and temperatures of 1,600-1,900° C. are expected as the demand for even more efficient engines continues. Cobalt and nickel base superalloys are used to form many gas turbine components, but even these superalloy materials must be aggressively cooled and/or insulated from the hot gas flow in order to survive long term operation in the combustion environment.
The high temperature ranges of future advanced turbine engines will require higher temperature capable materials such as ceramic matrix composites (CMCs). Simultaneously, a coolant usage reduction is being required for these advanced engines while at the same time the temperature of the available coolant is rising, making it more difficult to cool metal structures. CMC materials with their higher temperature capability will still require cooling but much less than needed for superalloys. Calculations, validated by rig testing, show that current oxide-based CMC materials with oxide-based thermal insulation (FGI: friable graded insulation) need less than 10% of the cooling air required for a superalloy. Nonoxide CMC materials with protective coatings need 20-30% of the cooling air required for a superalloy, which is 2×-3× the cooling air needed for an oxide-based CMC material system.
Ceramic matrix composite materials have many potential applications in high temperature environments due to their ability to withstand and operate at higher temperatures and with less cooling than that of current superalloy parts in gas turbines. However, CMC's can survive temperatures in excess of 1,200° C. for only limited time periods in a combustion environment due to environmental degradation caused by the presence of water vapor in the combustion gas stream. Furthermore, nonoxide-based CMCs are also subject to oxidation that further limits their useful life. Non-oxide based CMCs can be aggressively cooled to withstand temperatures above 1200° C., but require a protective environmental barrier coating (EBC) system because they are subject to both oxidation attack and environmental degradation due to water vapor that limits their useful life.
Current nonoxide CMC technology for gas turbine engines relies on silicon-based materials. Silicon-based non-oxides such as silicon carbide (SiC) and silicon nitride (Si3N4) are subject to both oxidation and attack by high temperature, high pressure water vapor in a combustion product environment. In this dual degradation mechanism, the silicon carbide or silicon nitride is oxidized to form a thermally grown oxide (SiO2) layer. This oxide layer then reacts with the high temperature, high pressure water vapor to form a volatile hydroxide species [Si(OH)x] which is then lost to the environment. Thus, surface recession occurs in a continual process as the protective SiO2 layer forms the hydroxide and volatilizes, and the new Si-based nonoxide ceramic surface oxidizes to replenish the lost SiO2. This process is enhanced by the high velocity gas stream in a gas turbine environment and is further enhanced at higher temperatures, pressures and water vapor contents.
Accordingly, multi-layer environmental barrier coating systems have been developed to protect silicon-based nonoxide CMCs. Typically these systems have a minimum of three layers on the nonoxide CMC substrate. These layers typically comprise a bondcoat layer, typically silicon, at least one intermediate layer to improve thermal expansion compatibility between the bond coat and the top coat, and lastly the top coat that provides some degree of water vapor degradation resistance. Such multilayer systems have been developed to protect silicon-based non-oxide ceramics from the combustion environment. U.S. Pat. No. 5,391,404 describes a process for coating a silicon-based ceramic with mullite, and U.S. Pat. No. 5,985,470 describes a barium strontium aluminosilicate (BSAS) bond coat underlying a thermally insulating top coat over a silicon carbide containing substrate. U.S. Pat. No. 6,969,555 B2 describes a multi-layer EBC system where the top EBC layer is an alkaline earth metal aluminate or a rare earth aluminate. These EBC's multi-layer systems typically function at a maximum surface temperature of 1,200-1,350° C. Since growth of a silicon dioxide layer underneath the multi-layer environmental barrier coating system could result in spalling of the coating and loss of environmental protection, the environmental barrier coating material must be sufficiently dense to prevent the ingress/diffusion of oxygen through the coating; for example having only closed porosity of no more than approximately 10% and having no open porosity.
Current oxide-based CMC's can not be cooled effectively with active cooling systems due to their low thermal conductivity and need a thermal protective layer for use at extended times (1000's of hours) above 1200° C., such as described in U.S. Pat. No. 6,013,592. This thermal protective layer known as FGI (friable graded insulation) is typically an alumina-mullite oxide material that is thermally stable in oxidizing environments up to about 1800° C. FGI can generally withstand combustion engine environments up to 1400° C. However, in combustion environments with high water vapor contents greater than about 10% and/or temperatures that exceed 1400° C., the FGI will be subject to degradation caused by the water vapor. The degradation mechanism is dependent on the operating temperature. Initially, degradation will occur from the water vapor reacting with the silica constituent of the mullite phase and forming a gaseous silicon hydroxide. This gas species is swept away by the gas stream and material is lost from the FGI. This loss of silica can weaken local areas of the FGI surface, which may be subjected to erosive forces from the high velocity gas stream. This process, over time, can result in recession of the FGI thermal protective layer. At higher temperatures, above about 1450° C., the alumina constituent of FGI may be subject to reaction with water vapor and the formation of a gaseous aluminum hydroxide species. This process will also contribute to recession/erosion of the FGI.
For each of the above degradation mechanisms, the specific rate of reactions are highly dependent on the combustion gas temperature, engine pressure, gas velocity, and the partial pressure of water vapor. The prevention, or minimization, of these mechanisms requires the use of some cooling (to reduce the reaction temperatures thus reducing the reaction rates) and/or the application of a protective coating, such as known hermetic environmental barrier coatings (EBC).